Conventionally, as sterilizing methods of medical instruments, high-pressure steam sterilization (hereinafter, simply referred to as “AC”) and ethylene oxide gas sterilization (hereinafter, simply referred to as “EOG sterilization”) have been widely used.
AC is a sterilization method in which an item to be sterilized is exposed under a high temperature at approximately 135° C., and has been widely used for medical instruments made of metal. However, there is a disadvantage that limitations exist in items to be sterilized since sterilization is performed under a high temperature condition. For example, there is a problem that heat labile materials such as plastics cannot be sterilized by AC.
On the other hand, EOG sterilization can be used for plastics since it can be performed at a lower temperature of 70° C. or below. However, due to its toxicity and risk of explosion, there is a disadvantage that EOG needs to be securely stored so as not to cause a problem associated with hygienics and safety, and sufficient care needs to be taken in handling. In addition, when EOG is supplied from a tank (cylinder) to a sterilizing apparatus via a pipe, the occurrence of weight reduction needs to be constantly monitored by measuring the weight of the cylinder for the purpose of preventing unexpected leakage from such as the pipes.
Besides those sterilization methods, a sterilization method using hydrogen peroxide (H2O2) has been used. As compared with EOG, hydrogen peroxide is simple to use and manage, and is useful from the safety perspective. However, since hydrogen peroxide is used in the form of an aqueous solution, the permeability to detail portions such as an inside of a tube is inferior to the AC or EOG sterilization.
As an alternative method to the AC or EOG sterilization, as shown in Japanese Unexamined Patent Publication No. 240864/1988, a sterilization method using high concentration ozone (O3) has also been used in which high concentration ozone is generated by providing a circulating pump in a position downstream from the ozone tank and upstream from the ozonizer and circulating ozone therethrough. In the method, an advantage exists that the generation of ozone and the decomposition of ozone after use are simple. However, there are disadvantages that high concentration ozone is explosive and gives the substantial damage to plastics.
As a sterilization method with no risk of explosion as compared with the above-mentioned various sterilization methods, a sterilization method using a nitrogen oxide gas (hereinafter, also simply referred to as “NOx”) has been proposed. In the method of Japanese Unexamined Patent Publication No. 162276/1983, for example, a gas mixture which is obtained by performing a plasma treatment to the gas mixture of oxygen and nitrogen is used for the purpose of sterilizing Escherichia coli present on such as food surface. In the method, a gas mixture of nitrogen oxide and ozone is prepared by performing the plasma treatment to a gas mixture introduced from an oxygen cylinder and nitrogen cylinder. The prepared gas mixture is sprayed on the surface of food to sterilize Escherichia coli present on the surface. Since the sterilization process can be performed at a moderate temperature, there are advantages that the method can be used for various items to be sterilized, and that sterilant gas does not need to be stored since nitrogen oxide is generated on demand.